We conducted a population genetic study of the western gall rust
fungus (WGR, Peridermium harknessii) using isozymes as genetic
markers. Electrophoresis of 341 single- gall aeciospore isolates
collected from several pine species revealed that WGR is comprised of
two distinct zymodemes (multilocus electrophoretic types) in the
western United
States. Within zymodemes, all 15 loci studied were monomorphic
(0.95 criterion), although variants were found at low frequency (<
0.03) at three loci. Zymodeme I was characterized by single bands,
indicating homozygosity at all loci; it was comprised of isolates
from all pine species and environments studied, including the Pacific
Coast and Cascade Ranges and the Sierra Nevada and Rocky Mountains.
Zymodeme II, which was absent from coastal forests, was characterized
by double or triple bands at six of 15 loci. The additional bands
were interpreted as products of alternative alleles in heterozygous
condition; isozyme phenotypes at the other nine loci were identical
to those of zymodeme I. The presumed heterozygotes were fixed within
zymodeme II, and homozygotes of alleles unique to this zymodeme were
not found. Generally, all isolates within a forest stand were in the
same zymodeme and, when isolates from both zymodemes were found in
the same location, recombinant genotypes between zymodemes were not
observed. Such extreme disequilibrium is inconsistent with sexual
reproduction, indicating that P. harknessii is asexual.